The stack consists of logical stack frames that are pushed when calling a
function and popped when returning. A stack frame contains the parameters to
a function, its local variables, and the data necessary to recover the
previous stack frame, including the value of the instruction pointer at the
time of the function call.
Depending on the implementation the stack will either grow down (towards
lower memory addresses), or up. In our examples we'll use a stack that grows
down. This is the way the stack grows on many computers including the Intel,
Motorola, SPARC and MIPS processors.

With that in mind our stack looks like this when function() is called (each
space represents a byte):
bottom of top of
memory memory
buffer2 buffer1 sfp ret a b c
<------ [ ][ ][ ][ ][ ][ ][ ]
top of bottom of
stack stack

As you can see, new (local) variables are pushed on top of the stack. Depending on the design of the architecture the stack grows towards higher memory addresses or towards lower memory addresses, the latter in your case.

From the viewpoint of the C language specification the order of memory locations of subsequently allocated variables is unspecified. Therefore, it depends ...